Efficacy of Transcutaneous Electric Nerve Stimulation on Parotid Saliva Flow Rate in Relation to Age and Gender.

STATEMENT OF THE PROBLEM
Treatment with salivary substitutes and stimulation of salivary flow by either mechanical or pharmacologic methods has side effects and only provides symptomatic relief but no long-lasting results.


PURPOSE
To assess the effectiveness of extraoral transcutaneous electric nerve stimulation (TENS) as a mean of stimulating salivary function in healthy adult subjects; as well as to determine the gender and age-dependent changes in salivary flow rates of unstimulated and stimulated parotid saliva.


MATERIALS AND METHOD
Hundred patients were divided into two groups; Group I aged 20-40 and Group II aged ≥ 60 years. The TENS electrode pads were externally placed on the skin overlying the parotid glands. Unstimulated and stimulated parotid saliva was collected for 5 minutes each by using standardized collection techniques.


RESULTS
Eighty seven of 100 subjects demonstrated increased salivary flow when stimulated via the TENS unit. Ten experienced no increase and 3 experienced a decrease. The mean unstimulated salivary flow rate was 0.01872 ml/min in Group I and 0.0088 ml/min in Group II. The mean stimulated salivary flow rate was 0.03084 ml/min (SD= 0.01248) in Group I, and 0.01556 ml/min (SD 0.0101) in Group II. After stimulation, the amount of salivary flow increased significantly in both groups (p< 0.001). Statistical comparison of the two groups revealed them to be significantly different (p< 0.001), with Group I producing more saliva. Gender-wise, no statistically significant difference was seen among the subjects in Group I (p = 0.148), and those in Group II (p= 0.448). Out of 12 subjects with 0 baseline flows, 7 continued to have no flow. Five subjects observed side effects, although minimal and transient.


CONCLUSION
The TENS unit was effective in increasing parotid gland salivary flow in healthy subjects. There was age-related but no gender-related variability in parotid salivary flow rate.


Introduction
Saliva is considered as a crucial fluid for maintenance of oral health and comfort. [1][2] Saliva has antibacterial, lubricant, remineralizing, digestive, soft tissue reparative, buffering, and cleansing properties. Therefore, decreased salivary production or altered salivary com-position may result in numerous clinical conditions that affect oral health, comfort, and quality of life.
Xerostomia is a real or perceived decrease in the amount of saliva. [1] Its prevalence in the general population is estimated to range 10-29%, and approximately 40% in adults above the age of 50. [3][4] It occurs more frequently in women than in men. [3] The main three causes of reduced salivary flow are medications, radiation therapy of head and neck cancers, and autoimmune disorders. [1] Treatment with salivary substitutes and stimulation of salivary flow by either mechanical or pharmacologic methods provide some symptomatic relief but no long-lasting result when active treatment is stopped. [5] Systemic sialogogues, like pilocarpine, work well in some patients; but their unavailability and side effects such as profuse sweating restrict their use. These drugs are also contraindicated in asthma, chronic obstructive pulmonary diseases, cardiac arrhythmias, and patients taking beta blockers. [1] Other treatments of xerostomia include daily gum chewing which leads to increased mastication and electrostimulation by using intraoral devices that yields moderate improvement. [6] High frequency, low intensity ultrasound therapy was also found to be ineffective in stimulating the salivary flow rates. [2] Recently, acupuncture treatment of patients with xerostomia [4,7] has been demonstrated to be effective and associated with long-lasting results. But the reluctance of patient to undergo "needle therapy" and unavailability of experienced acupuncturist make this treatment modality difficult. To overcome this shortfall, non-invasive transcutaneous electric nerve stimulation (TENS) device has been used to replace the needles.
As per our knowledge and literature search, only one pilot study has been conducted so far determining the efficacy of TENS in stimulating parotid salivary flow; but there is no study determining its effect in relation to age and gender. Thus, research in this area is minimal and sparse.
The purpose of the present study is to evaluate the effectiveness and safety of recently-developed extraoral TENS on parotid saliva flow rate. The study also determines any gender or age-dependent changes in unstimulated and stimulated parotid salivary flow rate in healthy population.

Materials and Method
In this prospective randomized comparative study, the subjects served as their own controls. The study was approved by the ethical clearance board of Kothiwal Dental College and Research Centre, Moradabad, Uttar Pradesh. A total of 100 subjects were randomly allocated into Groups I and II, irrespective of gender. The subjects in Group I were aged 20-40, and in Group II, they were ≥ 60 years. Informed written consent was taken from all patients.  The duct orifices on both sides were checked to lie over the inner ring of the cups. Unstimulated saliva was collected into vials for 5 minutes. The cups were then removed from the oral cavity; the remaining saliva was also collected from the tubing in the same vial. The cups were again replaced, the TENS unit was then activated.
The pulse rate was fixed at 50 Hz, the pulse duration at 250 µsec, and the unit was in normal mode. The intensity control switch was adjusted for patient comfort. Intensity was turned up 1 increment at a time at 5-second intervals until the subject raised their hand to indicate that an optimal intensity level was reached. Optimal intensity was defined as the maximum intensity that the subject still perceived to be comfortable. Stimulated saliva was then collected into a separate vial for 5 minutes ( Figure 2). were represented in number (%) and mean±SD (standard deviation). A log of adverse events was kept. Student's t-test was used to compare the groups. Correlation analysis was performed to assess the relationship between measurements. For all the tests, p≤ 0.05 was considered to be statistically significant.

Results
The in Group II the mean value was 0.0440 ml (Table 1).
In both groups, the minimum and maximum amount of flow noted in a patient was, respectively, 0 and 0.22 ml. Irrespective of gender, the mean unstimu-   Figure 3a).
In both groups, the males produced more saliva than females; however gender-wise, no statistically sig- There was great variability in the amount of saliva produced in our study. Some of the subjects demonstrated no flow initially. This was not surprising, as 21-22% of the population demonstrate no parotid flow even when measured over 5 minutes. [8] The wide variation of salivary flow rates in this study was within normal limits as reported in the literature. [6,8] The subjects that seemed to demonstrate significant change were those with initial saliva flow already present. In 7 out of 12 cases, the TENS was unable to stimulate saliva where the salivary flow was 0 at baseline. Since, it is the serous component of parotid saliva that confers the greatest protection against dry mouth; one may claim that TENS would not be useful. These findings also suggest that TENS, by itself, is less likely  to be effective in cases where there is no baseline saliva flow such as in long-standing Sjögren's syndrome or high-dose radiation therapy where complete destruction of the salivary gland unit has occurred. This is a shortcoming that would concern other current treatment modalities.
Meanwhile, in cases with residual salivary function, the TENS appears to be effective. These findings are suggestive that TENS may work quite well or even synergistically with other sialogogues. TENS may act more efficiently as an accelerator of salivary flow rather than an initiator. Therefore, it is likely to be more effective in cases of decreased salivary gland function rather than absolute absence of function.
In 3 subjects, salivary flow decreased with TENS, and in 3 others, it remained the same. The mechanism for this may involve the frequency and intensity settings and whether the brain perceived the stimulus as being painful. Typically, the salivary reflex is enhanced when nociceptive input reaches the brain via the trigeminal sensory nuclei. [9] However, not all preganglionic parasympathetic fibers are necessarily facilitated, some may be inhibited. This study did not evaluate the intensity and frequency that produces the maximum volume of saliva. We attempted to minimize these effects by keeping the stimulus at a tolerable level.
The effectiveness of TENS in stimulating salivary flow depended on age. Although, the literature has shown that salivary flow does not diminish with age, [6] our results are not in agreement with this observation.
There was no statistically significant difference between genders as reported in previous studies. [10] Preceding investigations do not support statistically significant gender differences in terms of salivary output even though females have a tendency to produce less saliva. The mechanism by which the TENS unit worked on the parotid gland is not clear. It is possible that it directly stimulated the auriculotemporal nerve that supplies secretomotor drive to the parotid gland. It is uncle- Chewing gum have shown mixed results in previous studies, [22] but needs to be avoided in those with temporomandibular disorders. Artificial saliva preparations are often objectionable.
One important shortcoming of the study is the placement of electrodes which was done approximately over the parotid region without exact anatomic measurements. But the diameter of the electrodes used in the study was large enough (6 cm) to overcome this limitation, as compared to the size of parotid gland.
To our knowledge, this is one of the few studies demonstrating the potential of TENS for increasing the salivary flow. Furthermore, our saliva collection method was more reliable than those used in previous electrostimulation studies, which were very subjective and prone to contamination by nasal and gastric secretions as well as food debris.

Conclusion
The